Not applicable.
Not applicable.
This invention relates to micromachined electromechanical systems (MEMS), and their use in switching technology for optical telecommunications.
Arrays of high speed, high precision actuation devices are becoming required for a proliferating number of applications, in diverse fields. Deep space astronomical observatories may use multifaceted mirrors, each facet independently controlled by a precision actuator. Digital projection cameras manipulate a plurality of reflectors, in order to cast an image onto a projection screen.
But optical telecommunications in particular, have an urgent need for high bandwidth, low inertia, microactuated reflectors. This industry uses optical wavelength radiation in communication channels, spanning long distances via optical fiber. The fiber generally carries single mode light, but with a bandwidth such that multiple frequencies can be transmitted by a single fiber strand. Called dense wavelength divisional multiplexing (DWDM), each of the multiple frequencies carries a single channel on a specific frequency within the fiber bandwidth.
A major technical challenge for optical telecommunications is switching the optical beams from a set of input fibers to a set of output fibers. Presently, this switching is achieved by converting the optical signal into an electrical signal, switching the signal electronically to the output circuit, and then converting the electrical signal back into modulated light that is inserted into an output fiber. This procedure has many disadvantages, including high cost, complexity, large footprint, and is relatively difficult to upgrade as bandwidths and frequencies change with time.
It is desirable therefore, to accomplish this switching without converting the light into electrical signals. An array of optical switches would address this application, wherein each optical switch (a micromirror) would redirect light from one of N input optical fibers into one of M output fibers. Nxc3x97M array of such switches would constitute an all-optical Nxc3x97M fabric for a telecommunications fiber network.
For such high speed, high precision applications, stringent design criteria are set on the physical and mechanical properties of the actuator. It should have low inertia and low power requirements. For low cost applications, it should also be mechanically simple. These considerations have led to the miniaturization of familiar electromechanical devices, using photolithographic processing rather than machining bulk components. Formation of sub-millimeter scale electromechanical systems is now well known in the art, as micromachined electromechanical systems, or MEMS.
Micromachined solenoidal magnetic actuators are known in the MEMS art as micro-solenoid switches. Typically, a slug of magnetic material is affixed to a piston or plunger, and a coil is provided whose diameter is sufficient to admit the slug into its interior. The coil is then energized to repel or attract the slug, depending on the direction of current in the coil. The resulting linear mechanical motion is used to actuate various linear devices, such as opening and closing a switch or valve, or driving a piston. An embodiment of a linear, solenoidal microactuator is found for example, in Guckel, et al., U.S. Pat. No. 5,44,177 (1997), xe2x80x9cMicromechanical magnetically actuated devices.xe2x80x9d
Another design option is a rotary actuator. This device resembles a miniaturized electromagnetic motor, with a ferromagnetic core material deposited on the substrate and wound with an electrical coil. The core is patterned with some arrangement of gaps, into each of which protrudes a driven member which interacts magnetostatically with the flux across the gap. A plurality of such elements, when driven in the proper sequence and timing, can produce a positive torque on a freely rotating member. Magnetostatic micromotors can be used as rotary actuators by mounting the device of interest onto the moving member, i.e. the rotor. This concept is clearly described in Mehregany, et al. in U.S. Pat. No. 6,029,337 (2000), xe2x80x9cMethods of fabricating micromotors with utilitarian features.xe2x80x9d
However each of these MEMS devices suffer from a common drawback, which is that the actuation motion is constrained by design and fabrication considerations, to be in the fabrication plane of the device. For example with solenoidal electromagnetic actuator, such as described in the aforementioned prior art, motion of the magnetic slug is required to be in the fabrication plane of the device. For the rotary micromotor, the device is mounted on the rotor, and so rotates in the plane of fabrication of the motor.
Many applications require motion in the orthogonal plane, i.e. vertical to the original plane of fabrication of the MEMS device, and an array of such devices is needed. Examples of such applications include ailerons composed of a multitude of fluid flow diverters, actuated or pivoting in the vertical direction relative to the array. Nxc3x97M optical switches require an array of independently addressable shutters or reflectors, in which the plane of actuation of the individual devices is orthogonal to the plane of the array, in order to intercept the beams of light passing over the horizon of the array. Projection cameras and multi-faceted reflectors require a tilting motion of the individual mirrors.
Therefore the MEMS fabrication substrate, cannot serve as the array plane for these applications, because the plane of actuation would be parallel to, not perpendicular to, the plane of the array.
Additional beams, gears and bearings can translate actuator motion out-of-plane, as in Ho et al., in U.S. Pat. No. 5,629,918 (1997), xe2x80x9cElectromagnetically actuated micromachined flap.xe2x80x9d In this invention a flap, which is the moving member of the actuator, is coupled by one or more beams to a substrate and thereby cantilevered out of the plan of the substrate. While conceptually this invention allows larger motions in out-of-plane directions, the need for multiple beams and pivots seriously complicates the design and fabrication of the device, and deleteriously affects tolerances and rigidity.
Therefore, an assembly of MEMS actuation devices, operating as an array, and with the plane of motion of each device being substantially perpendicular to the plane of the array, is not heretofore known in the art. Accordingly, there is a distinctly felt need for such an assembly of MEMS devices in a wide variety of applications, and in particular, within the optical telecommunications industry.
The invention described herein is an assembly method and apparatus used to mount a plurality of MEMS devices into an array. Each MEMS device is a low inertia, high bandwidth microactuator carrying the device of interest (shutter, piston, optical element, etc.). In the embodiment described here, each MEMS actuator carries an optical micromirror on the actuator arm. A plurality of like devices is fabricated on a silicon substrate, using processes known in the MEMS art.
Each of the devices is separated into individual dies after fabrication, by sawing the wafer into rows, and sawing the rows into individual dies. The dies are then individually mounted and adjusted on another substrate using an apparatus equipped with vertical and azimuthal actuation means. This second carrier wafer serves as a miniature optical table for the dies.
To achieve the desired plane of motion, the dies are rotated out of their original plane of fabrication, before being affixed to the second carrier wafer. In the preferred embodiment, a 90 degree rotation transforms the original plane of motion into one orthogonal to the plane of the array, as desired. Pine adjustment of the positioning is then accomplished with a feedback mechanism which optimizes the placement before the die is affixed to the second carrier wafer.
The carrier wafer has previously been photolithographically processed using techniques well known in semiconductor device fabrication, to form the supporting electronic circuitry to address and control each MEMS device. This second wafer, carrying the electronic circuitry, is henceforth termed the xe2x80x9ccircuit waferxe2x80x9d, as to distinguish from the original MEMS fabrication wafer.
As a result, the finished circuit wafer comprises an array of micromechanical reflectors, which each reflector independently addressable by associated circuitry. The array of reflectors in designed for use in the optical telecommunications industry, with each mirror capable of extension and retraction out of the plane of the array. This invention gives rise to the following significant advantages:
1. Ability to assemble switching devices in which the plane of the mirror is in the MEMS fabrication plane (which provides the highest quality, lowest loss mirror), but the mirror is deployed in the transverse plane, to intercept beams of light.
2. Ability to pre-test individual dies or rows and assemble only properly functioning devices (xe2x80x9cyielded diesxe2x80x9d). This is in contrast to optical switch arrays built on a wafer in which a non-yielding die means an imperfect assembled device. Such arrays consequently have either very demanding yield requirements or must be designed with sufficient duplicity to overcome the yield problems.
3. Ability to assemble optical switches with precise alignment independent of warping of substrate materials, and various other manufacturing defects, using a feedback mechanism during assembly.
4. Ability to use one type of optical switch for a large number of different assemblies for different applications. For example, this includes the ability to assemble Nxc3x97M switch arrays where N and M are any positive integer, using the same building block die.
5. Ability to assemble more than one type of die into an assembly of dies.
6. Ability to assemble a packaged device that is hermetically sealed, the hermetic seal can be confirmed, and the gas enclosed is such that the cooling of the device inside the package is optimized or the mechanical damping of the device is optimized.
The method and apparatus for constructing this array is the subject of this invention disclosure.